1. Field of the Invention
The present invention relates to a telecommunication system for selecting an optimum transfer rate, and more particularly to a system in a network using a common channel signaling system for switching information transfer rates.
2. Description of the Related Arts
Since an exchange of call setup information is effected via a common control channel in an interface taking a common channel signalling system, it is possible for a high speed digital trunk line to connect simultaneously a plurality of calls at different transfer rates on an interface. Since traffic amount of a high speed digital trunk line changes with time, communication equipment utilizing thereof communicates at a high speed and a high quality mode in the case of a small amount of traffic, whereas, when the traffic carrying capacity is large, a compact and flexible facility is desired so as to communicate therewith at a reduced communication speed in order to decrease the probability that a call is not completed because of a lack of channels in the trunk line.
In a prior art system, a call setup is attempted from an outgoing communication terminal at the highest transfer rate and if the communication at the highest transfer rate is not possible, the transfer rate is reduced in order and a process is initiated to connect the call again. Therefore, when a call is connected via a plurality of exchange units, there is the problem that it could take a long time before the call is connected. More particularly, when a call is broken up or released on account of a lack of available channels in the trunk line at a time that a SET-UP message arrives up to immediately before a terminating data terminal, it is disadvantageous in that a large amount of time is lost.
In an interface, and in conformity with the ISDN, when the standardized transfer rate is a first order group interface of 1.5 Mbps, it is limited to 64 Kbps, 384 Kbps, and 1.5 Mbps. Therefore, for example, in the case of a communication of 768 Kbps, a channel of 1.5 Mbps must be utilized, thereby reducing the utility efficiency of the trunk line.
The present invention is intended to solve the above problem.